Microbiology chapters
Malaria
The malaria parasites (P. vivax, P. falciparum, P. malariae, P. ovale, P. knowlesi), the life cycle in mosquito and human, why vivax relapses and falciparum kills, clinical features and severe malaria, laboratory diagnosis by thick and thin films and rapid tests with parasite counts worked out by hand, and treatment and prevention in Pakistan.
- Malaria is caused by Plasmodium spread by the bite of the female Anopheles mosquito (also by transfusion and congenitally); the mosquito is the definitive host (sexual cycle: sporogony) and humans the intermediate host (asexual cycle: schizogony); sporozoites infect liver cells (exoerythrocytic stage; P. vivax and P. ovale leave dormant hypnozoites that cause relapses), then merozoites invade red cells (rings → trophozoites → schizonts), whose synchronous rupture causes the fever paroxysm (every 48 h for vivax and ovale, 72 h for malariae); gametocytes taken up by mosquitoes continue the cycle.
- P. vivax is the commonest species in Pakistan (enlarged red cells, Schüffner dots; Duffy antigen needed); P. falciparum causes severe malaria because it infects red cells of all ages (very high parasitaemia) and sticks to small-vessel endothelium (PfEMP1, knobs) → cerebral malaria, severe anaemia, hypoglycaemia, acidosis, kidney failure, ARDS, blackwater fever; films show many small rings (often double, appliqué) and banana-shaped gametocytes.
- Diagnosis: thick film (detects parasites: sensitive) and thin film (identifies species) stained with Giemsa, or rapid diagnostic tests (HRP2 for falciparum, pLDH); treatment: chloroquine + primaquine (radical cure; check G6PD) for vivax, artemisinin combination therapy for uncomplicated falciparum and IV artesunate for severe malaria; prevent with insecticide-treated nets, indoor spraying and removing breeding sites.
On this page
The parasites
| Species | Notes |
|---|---|
| P. vivax | Commonest in Pakistan and South Asia; hypnozoites → relapses; prefers young red cells (reticulocytes); needs the Duffy antigen to enter red cells (Duffy-negative people, common in West Africa, are resistant); usually benign but can be severe (anaemia, splenic rupture) |
| P. falciparum | Most dangerous; infects red cells of all ages (very high parasitaemia); no hypnozoites (no true relapse, but recrudescence if treated inadequately); drug resistance (chloroquine resistance widespread) |
| P. ovale | Like vivax (hypnozoites); Africa |
| P. malariae | Quartan (72-hour) fever; old red cells; can persist for decades; nephrotic syndrome in children |
| P. knowlesi | Monkey malaria in South-East Asia; 24-hour cycle; can be severe |
Life cycle
In the human (intermediate host): asexual cycle (schizogony)
- Infected female Anopheles bites (night-biting) and injects sporozoites with saliva.
- Sporozoites reach the liver within an hour and invade hepatocytes.
- Exoerythrocytic (liver) schizogony: each sporozoite becomes a schizont releasing thousands of merozoites (about 1–2 weeks; this is the incubation period, with no symptoms).
- P. vivax and P. ovale: some sporozoites become dormant hypnozoites that wake weeks to months later → relapses. Only primaquine (or tafenoquine) kills them.
- Erythrocytic schizogony: merozoites invade red cells → ring form (early trophozoite) → trophozoite → schizont (with merozoites) → red cell ruptures, releasing merozoites (and pyrogens) → fever paroxysm; merozoites invade new red cells.
- Some merozoites become male and female gametocytes.
Periodicity (when cycles become synchronous):
- Tertian (every 48 hours): P. vivax, P. ovale ("benign tertian"); P. falciparum (often irregular or daily: "malignant tertian").
- Quartan (every 72 hours): P. malariae.
In the mosquito (definitive host): sexual cycle (sporogony)
Mosquito takes up gametocytes → fertilisation in the gut → ookinete → oocyst on the gut wall → sporozoites migrate to the salivary glands (about 10–14 days) → infectious bite.
Sporozoite infects humans; gametocyte infects mosquitoes. Mosquito = definitive host (sexual); human = intermediate host (asexual).
Other routes: blood transfusion, shared needles, transplacental (congenital malaria): these give no liver stage, so no relapse.
Clinical features
- Paroxysm: cold stage (shivering, rigors), hot stage (high fever, headache, vomiting), sweating stage (fever falls); then well until the next cycle.
- Anaemia, splenomegaly, hepatomegaly, mild jaundice, thrombocytopenia.
- In non-immune people (children, travellers) fever is often irregular; always test any febrile patient in an endemic area.
Severe falciparum malaria
Why falciparum is dangerous:
- Invades red cells of all ages → very high parasitaemia (can exceed 10%).
- Infected red cells develop knobs carrying PfEMP1, which stick to endothelium of small vessels (cytoadherence, sequestration) in the brain, kidneys, placenta and gut, and to uninfected cells (rosetting) → microvascular obstruction, local hypoxia and inflammation (TNF).
- Mature stages hide in vessels, so the peripheral film shows mainly rings (plus gametocytes).
Features of severe malaria (WHO, any one):
- Impaired consciousness / cerebral malaria (coma, seizures), prostration.
- Severe anaemia (Hb < 5 g/dL in children, < 7 g/dL in adults).
- Hypoglycaemia (also caused by quinine and pregnancy).
- Metabolic (lactic) acidosis, respiratory distress.
- Acute kidney injury; haemoglobinuria (blackwater fever: massive intravascular haemolysis with dark urine).
- Pulmonary oedema / ARDS; shock (algid malaria).
- Jaundice, abnormal bleeding (DIC).
- Hyperparasitaemia (above about 10%).
Malaria in pregnancy: placental sequestration → low birth weight, miscarriage, stillbirth, maternal anaemia and severe disease.
Protective genetic traits
- Sickle cell trait (HbAS): infected cells sickle and are removed.
- G6PD deficiency, thalassaemia traits, HbC, HbE, ovalocytosis.
- Duffy-negative red cells: resist P. vivax.
Laboratory diagnosis
| Test | Details |
|---|---|
| Thick blood film | Red cells lysed; examines a large volume of blood → detects parasites (more sensitive); quantifies parasitaemia (parasites per 200 or 500 white cells) |
| Thin blood film | Fixed with methanol; red cells intact → identifies species, shows red cell changes, % parasitaemia |
| Stain | Giemsa (pH 7.2) or Field's / Leishman |
| Rapid diagnostic tests (RDTs) | Antigen lateral-flow tests: HRP2 (P. falciparum-specific; can stay positive for weeks after cure; hrp2 deletions cause false negatives), pLDH (pan-species or species-specific), aldolase; quick, no microscope needed |
| PCR | Most sensitive; low-density and mixed infections; research and reference |
| Blood count | Anaemia, thrombocytopenia; QBC (fluorescent) method |
Repeat films every 12–24 hours (three times) if the first is negative but malaria is suspected.
Species on the thin film
| Feature | P. vivax | P. falciparum | P. malariae | P. ovale |
|---|---|---|---|---|
| Infected red cell | Enlarged, pale | Normal size | Normal or small | Enlarged, oval, fimbriated edges |
| Dots | Schüffner dots | Maurer clefts | Ziemann dots (fine) | James dots (Schüffner-like) |
| Rings | Single, large | Small, many per cell, double chromatin dots, appliqué (accolé) forms | Single | Single |
| Trophozoite | Amoeboid | Rarely seen in blood | Band form | Compact |
| Schizont (merozoites) | 12–24 | Rarely seen (8–24) | 6–12 ("rosette") | 6–12 |
| Gametocyte | Round | Banana / crescent-shaped | Round | Round |
| All stages in peripheral blood? | Yes | No (mainly rings and gametocytes) | Yes | Yes |
Parasites/µL = (parasites counted ÷ WBCs counted) × patient's WBC count per µL
(If WBC unknown, WHO uses 8,000/µL)
Counted 300 parasites against 200 WBCs, WBC = 8,000/µL:
300 ÷ 200 × 8,000 = 1.5 × 8,000 = 12,000 parasites/µL
Percentage parasitaemia from a thin film:
(infected RBCs ÷ total RBCs counted) × 100
60 infected in 2,000 RBCs → 60 ÷ 2,000 × 100 = 3%
Converting: RBC ≈ 5,000,000/µL; 3% of 5,000,000 = 150,000 parasites/µL
(above about 10%, or about 500,000/µL, counts as hyperparasitaemia)
Treatment (summary)
- P. vivax / P. ovale: chloroquine (blood stage) + primaquine for 14 days (radical cure; test G6PD first; avoid in pregnancy and infants).
- Uncomplicated P. falciparum: artemisinin-based combination therapy (ACT): artemether-lumefantrine (or artesunate + sulfadoxine-pyrimethamine, used in Pakistan's programme) + single low-dose primaquine to block transmission.
- Severe malaria: IV (or IM) artesunate (first choice; lower mortality than quinine), then a full ACT course; supportive care (glucose, fluids, transfusion, seizures, renal support).
- Drug details: antimalarial drugs.
Prevention and control
- Vector control: long-lasting insecticide-treated nets, indoor residual spraying, removing breeding sites (stagnant water), larvicides, larvivorous fish, screens.
- Personal protection: repellents, long clothing, avoiding outdoor exposure at night.
- Chemoprophylaxis for travellers: doxycycline, atovaquone-proguanil or mefloquine (chloroquine only where sensitive); start before travel and continue after return.
- Early diagnosis and full treatment (also reduces transmission).
- Screening blood donors in endemic areas.
- Vaccines: RTS,S and R21 (falciparum) are recommended by WHO for children in African endemic areas.
Pakistan: malaria is endemic, mostly P. vivax with a smaller share of P. falciparum, concentrated in Balochistan, Sindh, Khyber Pakhtunkhwa and the tribal districts; cases surged after the 2022 floods.
Expect: vector and hosts, life cycle stages (sporozoite, liver schizogony, hypnozoite, erythrocytic schizogony, gametocyte, sporogony), which species relapse and why, periodicity, why falciparum is severe (all ages of RBC, cytoadherence, PfEMP1), severe malaria criteria, blackwater fever, quinine hypoglycaemia, species features on blood films (Schüffner dots, banana gametocytes, band forms), thick vs thin films, RDT antigens, parasite density calculation, protective traits (sickle trait, Duffy), and treatment principles (chloroquine + primaquine, ACT, IV artesunate).
Quick revision
- Female Anopheles; mosquito definitive host, human intermediate.
- Sporozoite → liver (hypnozoites in vivax/ovale) → merozoites → RBC cycle → gametocytes.
- Tertian 48 h (vivax, ovale, falciparum); quartan 72 h (malariae).
- Vivax: commonest in Pakistan; enlarged RBC, Schüffner dots; Duffy antigen; relapses → primaquine.
- Falciparum: all RBC ages, cytoadherence (PfEMP1), cerebral malaria; multiple rings, banana gametocytes.
- Thick film detects; thin film identifies species; RDT HRP2 for falciparum.
- Treatment: chloroquine + primaquine (vivax), ACT (falciparum), IV artesunate (severe).
Written by The House Job from standard microbiology and parasitology references (Levinson's Review of Medical Microbiology and Immunology) and WHO malaria guidance.
FAQ
Which Plasmodium species cause relapses and why?
P. vivax and P. ovale, because they leave dormant hypnozoites in the liver that can reactivate weeks to months later.
Why is P. falciparum more dangerous than other species?
It infects red cells of all ages, producing very high parasite levels, and infected cells stick to small blood vessels in the brain and other organs, causing cerebral malaria and organ failure.
What is the difference between thick and thin blood films?
A thick film examines more blood and is better for detecting parasites and counting them; a thin film keeps red cells intact and is used to identify the species.
Which genetic traits protect against malaria?
Sickle cell trait, G6PD deficiency, thalassaemia traits and, for P. vivax, Duffy-negative red cells.